The present invention relates to a load weight measuring device for a multi-stage mast forklift truck.
A forklift truck includes a mast assembly having a mast unit, a lift bracket, forks attached to the lift bracket, and a lift cylinder unit for raising the lift bracket along the mast unit. There has been a demand for measuring the weight of a load while the load is being lifted by the forks. When the forklift truck is traveling with a load raised to a high position by the forks, various controlling operations are performed corresponding to the weight of the load in order to secure the stability of the forklift truck. A load weight measuring device used for such purpose is disclosed in Japanese Patent Application Publications No. 2000-16795 and No. 10-265194.
The load weight measuring device disclosed in the above-indicated Publications includes a mast assembly. Referring to FIGS. 14 and 15 showing the conventional forklift truck according to the above-indicated Publications, the mast assembly 100 has a multi-stage mast unit including outer masts 90 supported by a body frame, and inner masts 92 vertically guided by the outer masts 90 for vertically guiding and moving a lift bracket 91. The mast assembly 100 has a lift cylinder unit having a pair of left and right lift cylinders 93, 94. As shown in FIG. 16, respective lift cylinders 93, 94 have cylinder bodies 93A, 94A fixed to the outer masts 90, oil chambers 93B, 94B formed in the cylinder bodies 93A, 94A and piston rods 93C, 94C fixed to the inner masts 92 and extendable from the cylinder bodies 93A, 94A. As shown in FIGS. 14 and 15, a pair of chain wheels 95 is mounted to the top of respective inner mast 92, and a pair of chains 96 is wound around the respective chain wheels 95. One end of the chains 96 are fixed to the outer masts 90, and the other end of the chains 96 are fixed to the lift bracket 91.
As shown in FIG. 16, the oil chambers 93B, 94B are connected to each other through an oil passage 97, which is connected to a flow regulator valve 98 for regulating the maximum flow rate of hydraulic oil. A pressure sensor 99 is disposed in the oil passage 97 for detecting the pressure of hydraulic oil. Reference numerals 80, 81, 82, 83 and 84 designate a hydraulic pump, an oil control valve, a drain passage, an oil tank, and a safety down valve, respectively.
The forklift truck further includes a controller having therein a memory and a calculator that form a part of the load weight measuring device. Since the mast assembly 100 has the single-stage lift cylinder unit having one pair of the lift cylinders 93, 94, the memory stores parameters only for the single-stage lift cylinder unit.
According to the forklift truck having such a load weight measuring device, when the lift cylinders 93, 94 of the mast assembly 100 are operated by the forklift truck operator so as to extend the piston rods 93C, 94C, the inner masts 92 are raised by the lift cylinders 93, 94 while the inner masts 92 are guided by the outer masts 90. Accordingly, the lift bracket 91 is raised at double speed, or at a speed that is twice as much as the speed at which the inner masts 92 are raised while the lift bracket 91 is guided by one inner mast 92. Load weight acting on the lift bracket 91 is transmitted to the hydraulic oil in the oil chambers 93B, 94B of the lift cylinders 93, 94, and hydraulic pressure in the oil chambers 93B, 94B is detected by the pressure sensor 99. The calculator calculates the load weight acting on the lift bracket 91 based on a pressure signal outputted from the pressure sensor 99 and the parameters stored in the memory. The data of calculated load weight is used for various purposes, such as displaying the value of calculated load weight on a display device, providing a warning signal when the calculated load weight exceeds a predetermined value, and controlling of the forward-tilting angle of the mast assembly 80 and the traveling speed of the forklift truck.
The above-described conventional load weight measuring device is used for a forklift truck having a mast assembly with a single-stage lift cylinder unit. If this load weight measuring device is used for a forklift truck having a mast assembly with a double-stage or multi-stage lift cylinder unit, the load weight measuring device cannot always measure the load weight correctly.
There are various types of mast assemblies, such as a mast assembly having a two-stage mast unit and a single-stage lift cylinder unit, a mast assembly having a two-stage mast unit and a two-stage lift cylinder unit, and a mast assembly having a three-stage mast unit and a two-stage lift cylinder unit. For example, there is a mast assembly having a two-stage mast unit and a two-stage lift cylinder unit, in which oil chambers of the lift cylinders of each stage are connected to each other in series from the flow regulator valve toward the downstream with respect to the direction in which hydraulic oil flows, and the lift cylinder having the oil chamber of the second stage is operated thereby to extend its piston rod firstly. This type of mast assembly is called a full free lift mast assembly. The full free lift mast assembly is operatable in such a manner that the lift bracket is raised firstly to the level of the top end of the inner masts while the inner masts of the second stage remains at its lowered position without moving up relative to the outer masts of the first stage, and then the inner masts are raised to the level of the top end of the outer masts. A forklift truck having such a full free lift mast assembly has some advantage when the forklift truck is used in a place whose ceiling is not sufficiently high. That is because the full free lift mast assembly enables the forklift truck to perform the operation of loading without causing a collision between the mast of the forklift truck and the ceiling. In the forklift truck having a full free lift mast assembly, the load weight acting on the lift bracket can be calculated by the load weight measuring device based on the parameters for the first-stage mast unit in the low lift stage of the mast assembly when the inner masts is not raised relative to the outer masts, and the lift bracket is raised relative to the inner masts. Meanwhile, in the high lift stage of the mast assembly when the inner masts are raised relative to the outer masts, the parameters for the first-stage mast is not appropriate for the high lift state, so that correct calculation of the load weight cannot be accomplished. Therefore, the value of the load weight shown on the display is incorrect, a warning signal is provided incorrectly, and the controlling of the forklift truck operation cannot be accomplished appropriately. This is true of a forklift truck having a mast assembly with a three-stage mast unit and a two-stage lift cylinder unit.
The mast assembly having a multi-stage mast unit and a multi-stage lift cylinder unit is a so-called full free mast assembly, such as a FV mast assembly, a FW mast assembly, a FSV mast assembly and an FSW mast assembly. As shown in Table 1, the FV mast assembly has a two stage lift cylinder unit having one pair of first lift cylinders and one second lift cylinder. The FW mast assembly has a two-stage lift cylinder unit having two pairs of first lift cylinders and second lift cylinders. The FSV mast assembly has a two-stage lift cylinder unit having one pair of first lift cylinders and one second lift cylinder. The FSW mast assembly has a two-stage lift cylinder unit having two pairs of first lift cylinders and second lift cylinders. Meanwhile, the V mast assembly having a two-stage mast unit and a single-stage lift cylinder unit is not the full free mast assembly.
TABLE 1MastNumber ofLift cylinderLift cylinderassemblyNumber of firstsecond liftoperated in theoperated in thetypelift cylindercylinderlow lift statehigh lift stateFV21SecondFirstFW22SecondFirstFSV21SecondFirstFSW22SecondFirstV2NoneFirstFirst
When the load weight measuring device is used for the mast assembly with the multi-stage lift cylinder unit, a detecting device detects a state which stage of the lift cylinder raises the lift bracket, then a selector is actuated to select parameters from the predetermined parameters to be used by a calculator, and the calculator can calculate the load weight based on the parameters for the detected stage lift cylinder unit.
The present invention which has been made in light of the above problems is directed to providing a load weight measuring device which is adapted for use in a multi-stage mast forklift truck having a mast assembly with a multi-stage lift cylinder unit having lift cylinders, and which can always measure the load weight correctly.